1. Field of the Invention
The present invention relates to an image shooting apparatus, and particularly to an image shooting apparatus having an auxiliary light emitting device that is used to detect focus on a low-light or low-contrast object.
2. Description of the Prior Art
Conventionally, automatic focusing (AF) has been widely used in cameras and similar apparatuses. Automatic focusing is achieved by the use of an automatic focus adjustment device that automatically adjusts focus by calculating the degree of defocus of the taking lens or by measuring the distance to the object. For example, in one type of automatic focus adjustment device, focus is adjusted by first calculating the degree to which the image of the object is out of focus (defocused) based oin the relative position or contrast of the image within a focus-detection area on the screen, and then driving the taking lens in accordance with the thus calculated degree of defocus. For a low-light object, focus detection is performed actively by illuminating the object with auxiliary light emitted from an auxiliary light source provided on the part of the camera.
Moreover, in some cases, to widen the field of view for distance measurement so as to securely catch the object to be targeted for distance measurement, a plurality of focus-detection areas are provided, a plurality of auxiliary light beams are emitted to the object, and a plurality of light-sensing devices are provided to sense the light reflected from the object. This serves to prevent shortage of reflected light that may result depending on the conditions in which the object is placed when distance measurement is performed by emitting a spot light to the object, or to prevent failure of or errors in distance measurement that may result, for example, when two persons to be photographed are standing side by side and the distance-measurement field of view, if it is relatively narrow, falls just between the two persons.
In some conventional examples, auxiliary light is emitted by simultaneously turning on a plurality of light-emitting diodes built into the camera body or flash. These light-emitting diodes, however, receive a current as low as about 200 mA even at the peak. and therefore they cannot be used for so-called low-contrast compensation as will be described later. The flash may incorporate a power source of sufficient capacity, but the use of such a power source is inadequate because, in a situation where the flash needs to be fired anyway, the object is in low-light conditions, and therefore there is no need to use high-intensity auxiliary light, that is, low-intenisity auxiliary light suffices for such a situation. In other conventional examples, the light emitted from one light-emitting diode is made into a slit-shaped beam of light through a prism so that all focus-detection areas will receive the light simultaneously. This method, however, not only necessitates development of special optical systems and special light-sensing devices, but also suffers from shortage of light.
The conventional devices and methods as described above, however, have the following disadvantages. If an auxiliary light device that uses a light source, such as light-emitting diodes (LEDs), built into the camera body is disposed too close to the main optical axis of the taking lens, the auxiliary light tends to be eclipsed by the taking lens. In particular, the part of the auxiliary light that strikes (among others) the focus-detection area that is situated opposite to the light source with respect to the main optical axis of the taking lens is most susceptible to eclipse. By contrast, if the auxiliary light device is disposed too far away from the main optical axis, the auxiliary light may be obstructed by the hand of the user of the camera. This cannot be avoided unless the camera body is made larger.
Moreover, where a plurality of beams of auxiliary light need to be emitted to a plurality of focus-detection areas, dividing the light from a single light-emitting diode that is built into a camera as an auxiliary light source leads to shortage of light. That is, it is almost essential to turn on a plurality of light-emitting diodes. In particular, where focus detection is impossible because the object is in well-lit but low-contrast conditions as when shooting a white wall, contrast needs to be enhanced (so-called low-contrast compensation) by the use of auxiliary light that is sufficiently intense relative to the illuminance of the object and that has a pattern. This, however, cannot be achieved without feeding each light-emitting diode with a current, for example, as high as about 600 mA.
When a plurality of light-emitting diodes are turned on simultaneously, they require too much current; for example, as few as two light-emitting diodes require as high a current as, for example, about 1.2 A. This is difficult to cope with considering the capacity of the power source that can be accommodated in a limited space secured for it within the camera body. Moreover, this may adversely affect the light-sensing devices and the controller. In addition, to prevent destruction of the light-emitting diodes that are fed with so high a current, it is necessary to limit the duration for which they are kept on and secure periods in which they are kept off for heat dissipation. This necessitates elaborate control of the light-emitting diodes such that they are turned on not simultaneously but successively and that their "on" and "off" periods are regulated.
On the other hand, conventionally, auxiliary light target areas are typically determined with respect to the above-mentioned focus-detection areas in such a way that the most effective illumination is achieved when a so-called standard lens is used as the taking lens. This means that, in shooting with a wide-angle lens, a recently developed zoom lens having a wide-angle region, or a similar lens mounted on the same camera, the focus-detection areas are disproportionately large with respect to the object and the auxiliary light target areas. In particular, the focus-detection areas far away from the main optical axis of the taking lens fall disproportionately far away from the main optical axis, causing off-axial auxiliary light beams to strike almost completely off their respective focus-detection areas.
For example, as shown in FIG. 17, the auxiliary light target areas are conventionally so determined as to agree with the focus-detection areas available with the standard lens, like the central, left-hand, and right-hand auxiliary light target areas 10, 12, and 13, respectively. However, when the lens is changed to a wide-angle lens, the central, left-hand, and right-hand focus-detection areas are located as indicated by 7w, 6w, and 8w, respectively, and thus, in particular, the left-hand and right-hand focus-detection areas 6w and 8w cannot be covered by the off-axial auxiliary light target areas 12 and 13 as determined in the conventional manner.
By contrast, if the auxiliary light target areas are so determined as to be entirely fit for the wide-angle lens, they are disproportionately large relative to the focus-detection areas when the standard lens is used. This reduces the illuminance of the object illuminated by the auxiliary light and thus makes the illumination range unduly short. In addition, this makes it impossible to maintain the conventional performance in cases where focus detection is impossible because the object is in well-lit but low-contrast conditions as when shooting a white wall and where contrast therefore needs to be enhanced (so-called low-contrast compensation) by the use of auxiliary light that is sufficiently intense relative to the illuminance of the object and that has a pattern.